They are the brutal terrorist organisation that claims religious authority over Muslims across the world.

The al-Qaeda offshoot is confusingly known by three names: Islamic State, ISIS and ISIL – but a terrorism expert says they have got their branding spot on.

The murderous group, which now has around 50,000 fighters in Syria and 30,000 in Iraq according to August 2014 figures from the Syrian Observatory for Human Rights, until recently used the full Arabic name Al-Dawla Al-Islamiya fi al-Iraq wa al-Sham - the Islamic State of Iraq and al-Sham.

In June, the terror group declared a caliphate - an Islamic state led by a supreme religious and political leader - and began using the simplified name 'Islamic State'.

‘ISIS is very smartly branding itself as THE Islamic State, that’s how it’s been able to attract so many followers,’ Dr Anne Aly, a Research Fellow at Curtin University, told Daily Mail Australia.

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Terrorist leader: The Islamic State's leader, also known as ISIS or ISIL, Abu Bakr al-Baghdadi has been presented as a rightful Caliph

‘It is differentiating itself from al-Qaeda by saying “we are IT, we are what you haven’t been able to achieve all this time”.’

The counter-terrorism expert explained that by using the name Islamic State people believe they are signing up to support a legitimate and organised group.

‘They don’t understand, they think they are going to join a humongous well organised army called ISIS to give out welfare. They think they are joining something like a state with a humanitarian wing.

‘But it’s not like that at all because there are lots of different competing factions within Syria… there is the Free Syrian Army… so you get two brothers going over there, both thinking they are fighting for the Islamic state, but they end up on two different teams shooting each other, killing each other.’

News organisations, presidents, prime ministers and other world leaders are currently using a mixture of three different names to reference the organisation which is essentially one movement.

The name ISIS – the Islamic State of Iraq and Syria - is used by American news organisations such as the New York Times and L.A. Times.

Fooled: People are joining the Islamic State in Syria and Iraq because they think it is a well 'organised army', ' Dr Anne Aly, a Research Fellow at Curtin University, said

Supporter: The Islamic State has captured swathes of territory in Iraq and Syria and has declared itself an Islamic Caliphate and called on factions worldwide to pledge their allegiance

Mission: The Islamic State are trying to create 'a state that does not recognise the previous borders of Iraq or Syria,' Dr Aly said

International publications such as The Guardian also use ISIS and the BBC uses a combination of ISIS and Islamic State.

In a recent report condemning the group, the United Nations referred to the group as ISIS and by its original name.

‘Forces of the Islamic State of Iraq and al-Sham have committed torture, murder, acts tantamount to enforced disappearance and forced displacement as part of attacks on the civilian population in Aleppo and Raqqa provinces, amounting to crimes against humanity,’ it said.

The U.S. government, including President Obama, the Pentagon, and the State Department, uses ISIL which stands for Islamic State of Iraq and the Levant.

The group has used this name because it aspires to control what used to be the historic region called the Levant. This area includes Jordan, Israel, Palestine, Lebanon, Cyprus, and an area in southern Turkey that includes Hatay.

However, the Islamic State’s ambitions are now even wider – it wants to bring much of the Muslim-inhabited regions of the world under its direct political control.

The Australian government, including Prime Minister Tony Abbott, also refers to the group as ISIL.

Beheadings: The Islamic State notoriously kills people by beheading them. Australian jihadist Mohamed Elomar is seen here holding up two severed heads in Syria

Control: The Islamic State flag (left) is waved by thousands of supporters in Syria and Iraq. This map (right) shows the areas controlled by the Islamic State in red, the areas claimed by the Islamic State in beige and the rest of Iraq and Syria in pale yellow

‘There are hundreds of Westerners fighting with extremist groups such as ISIL – including at least 60 Australians in Syria and Iraq, and about 100 facilitators in Australia. That means we could be confronting more potential home-grown terrorists than ever before,’ Mr Abbott and Attorney General George Brandis said in a joint statement on 26 August.

Similarly the UK government and Prime Minister David Cameron use ISIL.

On 27 August, Mr Cameron’s office issued a statement after the brutal beheading of American photojournalist James Foley by the terror group.

‘The Prime Minister has condemned the barbaric and brutal murder of James Foley by ISIL (Islamic State of Iraq and the Levant) in Syria. This will not change our commitment to tackling this appalling organisation and doing everything we can to keep our country safe,’ it said.

Murdered: American photojournalist James Foley was killed by an Islamic State terrorist in Syria. The group posted a graphic video of his murder online, nearly two years after he was first kidnapped, in August 2014

Destroyed: In this photo people walk through the rubble of a Shiite mosque after it was destroyed in a bomb attack by militants of the Islamic State, formerly known as the Islamic State in Iraq and the Levant (ISIL) in the city of Mosul, July 23, 2014

‘The situation in Northern Iraq remains deeply worrying and we condemn the barbaric attacks waged by ISIL terrorists across the region. Around 500,000 people are displaced in Dahuk province alone and the United Nations has now declared the situation as the highest level of humanitarian emergency,’ it continued.

Dr Aly said whether you use ISIL, ISIS or Islamic State is neither ‘here nor there’ but the fact they are calling themselves an Islamic State is important.

‘It is significant because what they are trying to create is a state that does not recognise the previous borders of Iraq or Syria. To say Islamic state is to symbolically say “we have created one state that does not respect the borders”.

‘It’s about a symbolic state - it also communicates their strategy or end goal of establishing true statehood in the region, which they are never going to get - let them dream,’ she said.

Dr Aly explained that the Islamic State’s leader- Abu Bakr al-Baghdadi – has been presented at a very important time as a rightful Caliph, the head of state in a Caliphate.

‘Baghdadi is being brought forward as the rightful and true Caliph. He claims to have the right lineage, the way he walks and talks are also important for people to pledge allegiance to him.

‘That’s how you create a state - when everyone supports him,’ she said.

Dr Aly said another big difference between al-Qaeda and other terrorist organisations in the region, compared to Islamic State, is that they never really paid attention to ‘having a valid and rightful caliph’.